Electrosurgical devices perforate or cut tissues when radio frequency (RF) electrical energy rapidly increases tissue temperature to the extent that the intracellular fluid becomes converted to steam, inducing cell lysis as a result of elevated pressure within the cell. The radio frequency range lies between 10 kHz and 300 MHz, but electrosurgical devices usually operate at a frequency between 400 kHz and 550 kHz. This technology can be used to create perforations in different types of tissue, such as heart tissue, vascular occlusions, and others. Commonly, RF devices are described for use in perforating vascular occlusions. A device to dilate and/or lance blood vessels that are morbidly contracted or clogged is described in a European Patent Application of Osypka, Publication Number EP 0315730, published May 15, 1989. This device describes the use of RF energy in either bipolar or monopolar application modes to open blood vessels by means of heat. Other devices intended to use RF energy to pass through occluded vessels have also been described (U.S. Pat. No. 5,364,393, of Auth et al., issued Nov. 15, 1994, WO 93/20747, publication of PCT Patent Application No. PCT/US93/03759, of Rosar, published Oct. 28, 1993, U.S. Pat. No. 5,098,431, of Rydell, issued Mar. 24, 1992, and U.S. Pat. No. 4,682,596 of Bales et al., issued Jul. 28, 1987). U.S. Pat. No. 6,293,945 B1, of Parins et al., issued Sep. 25, 2001 describes an electrosurgical instrument with suction capability. This device has three functions at the tip including cutting, coagulating, and suction. None of these devices however incorporate a means for verifying the location of the device within the body. One means for verifying location is described in U.S. Pat. No. 4,936,281, of Stasz, issued Jun. 26, 1990, which describes an ultrasonically enhanced RF catheter used for cutting. An ultrasonic transducer connected to an electronics module receives echo signals, enabling Doppler flow readings and ultrasound imaging of the vessel.
Having reliable information about the location of electrosurgical devices within a heart is an important aid to performing a successful procedure. It is often valuable to have more than one source of this information because every imaging technique has limitations, and using only one method can lead to erroneous information. A number of different tools and techniques have been used to assist the physician in locating devices within a heart. These include the use of fluoroscopy, cardiac pressure monitoring, transesophageal echocardiography, intracardiac echocardiography and the use of other devices in the heart to identify certain landmarks. L. J Jordaens et al. (2001) states that intracardiac echocardiography provides potentially useful information for electrophysiological studies and pacemaker implantation by visualization of important anatomical landmarks and structures.
Monitoring thy FCG tracings from an intracardiac surgical device can he a useful tool to verify the position of the device in a heart. An electrode with reference to a ground or a pair of electrodes placed directly on or in the heart will record a repeating pattern of changes in electrical action potential. Action potential can be defined as an explosion of electrical activity that is created by a depolarizing current within biological cells. As action potentials spread from the top chambers of the heart (atria) to the bottom chambers of the heart (ventricles), the voltage measure between a single electrode and a ground or a pair of electrodes will vary in a way that provides a picture or electrocardiogram (also referred to as electrograms), of the electrical activity of the heart. The nature of this picture can he varied by changing the position of the recording electrode(s); different locations in the heart are known to have characteristic ECG tracings or measurements. A bipolar recording is the voltage measurement between two electrodes and a unipolar recording is the voltage measurement between a single electrode and an electrode that is attached to a patient or an electrode that is built into a recorder or electrocardiograph and maintained at zero potential (ground). J. A Alvarez et al. (1991) who conducted experiments where ECG signals were monitored in a heart using a transseptal needle states that the endoatrial electrocardiogram registered while the needle (Brockenbrough transseptal needle) pressed muscular areas of the septum or the free atrial wall showed marked injury curves; on the other hand, no significant changes (in the endoatrial electrocardiogram) were observed at the area assumed to be the fossa ovalis floor. This implies that the ECG tracing observed when a surgical device is in contact with the fossa ovalis which is a membranous region will be markedly different from ECG tracing observed on the muscular areas of the atrial septum or the free atrial wall. This difference in tie electrocardiogram may be used to locate a surgical device on the region of the fossa ovalis in a heart.
Knowing the electrical action potential or ECG at the tip of a perforation device is a useful tool to determine the location of the device, particularly in instances where imaging techniques provide inconclusive information. A device that is used for monitoring and recording the electrical activity of the heart is described in U.S. Pat. No. 4,892,104, of Ito et al., issued Jan. 9, 1990; however the device is not capable of perforating tissue using RF energy. U.S. Pat. No. 6,296,615 B1, of Brockway et al., issued Oct. 2, 2001, describes a catheter with a physiological sensor. This catheter consists of a pressure transducer for monitoring pressure, as well as the ability to detect and/or transmit an electrical signal.
It is often required to create a perforation in the atrial septum to gain access to the left side of the heart interventionally to study or treat electrical or morphological abnormalities. It is also often desirable to create a hole in the septum in order to shunt the blood flow between the left and right sides of the heart to relieve high pressure or provide more blood flow to certain areas. The location on the atrial septum where a perforation is most commonly created is the fossa ovalis. The fossa ovalis is an oval depression in the inferior part of the interatrial septum. It represents the foramen ovale of the fetus and is generally a thin membraneous structure. Since the atrial septum is muscular in nature, it is generally easier to create a perforation across the fossa ovalis to gain access to the left side of a heart. Historically in these instances, a dilator and guiding sheath are introduced into the femoral vein over a guidewire and advanced into the right atrium. The guidewire, dilator and guiding sheath are usually packaged as a kit with the guiding sheath designed to track over the dilator. In most designs, the distal end of the dilator extends out beyond the distal end of the sheath once the two devices are locked together. Once the dilator and guiding sheath are positioned appropriately in the right atrium, a stiff needle such as the Transseptal needle of Cook Incorporated, Bloomington, Ind., USA is introduced through the dilator and guiding sheath set in the femoral vein and advanced through the vasculature into the right atrium. From there the needle tip is positioned at the fossa ovalis, the preferred location on the septum for creating a hole. Once in position, mechanical energy is used to advance the needle through the septum and into the left atrium. Once in the left atrium the needle can be attached to a pressure transducer and the operator can confirm a left atrial pressure before continuing with the procedure. An operator may dilate the hole by advancing the dilator over the needle into the left atrium and tracking the guiding sheath over the dilator and into the left atrium to provide access for other devices to the left heart once the needle and dilator are removed. As well, the operator may use another device such as a balloon catheter delivered over a guidewire to enlarge the hole made by the needle if a shunt between the right and left atria is desired.
Another device and method for creating a transseptal puncture is described in U.S. Pat. No. 5,403,338, of Milo, issued Apr. 4, 1995, which describes a punch that is intended to create an opening between two compartments. This device also makes use of mechanical energy, as with the transseptal needle.
These methods of creating a transseptal perforation rely on the skill of the operator and require practice to be performed successfully. The needles used in this procedure are very stiff and can damage the vessel walls as they are being advanced. In addition, the amount of force required to perforate the septum varies with each patient. If too much force is applied there is the possibility of perforating the septum and continuing to advance the needle so far that damage is done to other areas of the heart. C. R. Conti (1993) discusses this possibility, and states that if the operator is not careful, the posterior wall of the heart can be punctured by the needle when it crosses the atrial septum because of the proximity of the two structures. It can also be difficult to position the needle appropriately in hearts that have malformations, or an atypical orientation. Justino et al. (2001) note that despite improvements to the technique with the needle since its first introduction, most large studies continue to report failed or complicated mechanical transseptal punctures, for reasons such as unusual septal thickness, or contour. Patients with a muscular septum, as well as those with a thick septum can benefit from an alternative to the transseptal needle puncture (Benson et al, 2002), as it is difficult to control the amount of mechanical force required to create the puncture. Furthermore, children born with heart defects such as hypoplastic left heart syndrome could benefit from an alternative technique. The abnormal anatomy of these patients including a small left atrium increases the likelihood of injury or laceration of surrounding structures during transseptal puncture (Sarvaas, 2002). The patient population discussed above would benefit from a device and technique for transseptal perforation that allows for a more controlled method of perforation and a method to confirm that the perforation has been made in the correct location.